[T-Mobile Samsung Galaxy S II SGH-T989] monkeybuttons:
First off, the motivation to do this came from these threads:

http://xdaforums.com/showthread.php?t=1610336

http://xdaforums.com/showthread.php?t=1441510

http://xdaforums.com/showpost.php?p=21795243&postcount=62

---------- Please read these threads and hit their authors Thanks buttons, because they are awesome! ----------

Background:

I have suffered from call echo on every AOSP ROM I have installed, and even on custom stock ROM's. I have the D30 phone case T-Mobile sells, so I always thought that was the problem. Without the case, stock ROM's didn't really echo (it was there, but only noticeable to those who were really annoyed by it when I was running AOSP ROM's, such as the wife). Also, when I would shift the phone in my hand, the person I called would ask me what I was doing because it sounded like I was beating the phone on a hard surface. It is very frustrating.

I always assumed my call echo couldn't possibly be a hardware issue since I have never dropped my phone, nor been overly rough on it. However, after reading the above threads (including ALL the comments - scary, I know) I decided this fix was worth a try.

Now, for the fix. I take no credit for it as I simply followed in the footsteps of those brave souls who attempted this on the international version of the SII. I also assume no responsibility for any damage to your phone due to the instructions or information contained in this thread. If your phone explodes, fries, dies, breaks, you cut yourself and bleed to death, you let the magic smoke out of the phone, the fix doesn't work for you --- etc., etc., etc. --- I take no responsibility.

First things first, you will need tools (duh), a business card, scissors, probably tweezers, a credit card to pry the back off the phone, and PATIENCE!

1. Shut down your phone and remove the battery, SIM card and sd card.

2. Using the right sized Philips screwdriver remove all six screws in the order shown in the first picture. (The smallest Philips screwdriver I have is a #0 which is not small enough. However, with great care I was able to remove the screws with a 1.4mm flathead screwdriver. I recommend finding the correct size screwdriver before you try to begin.) Remember which screws went where. In case you forget or they somehow get mixed up, screws numbered 1 and 2 in the first picture do NOT have Locktite on them, the rest do.

3. Locate the prying points along the edges of the phone and use your credit card (or other sturdy, non-metallic object) to pry to clips loose. A few of the prying points are marked with arrows in the first picture. They all look similar. I recommend starting at the corner where screw #1 is and go clockwise around the edge of the phone until all but the top edge has been popped loose (unclipped). Picture 2 shows why you shouldn't try to unclip the top of the phone. There are three tabs that would be nearly impossible to pry loose without breaking them.

4. Starting at the bottom of the phone, near the speaker, carefully lift up the backing of the phone. There is a snap between the camera and flash that retains the backing. It is difficult to free it, so work slowly, keep steady pressure on it, work it back and forth, up and down, side to side until it pops loose. Again, I take no responsibility if you break the plastic backing of your phone. That said, the backing is fairly tough and can withstand moderate force (fingernail strength would probably be a good estimate of how hard you can pull on it).

5. There are no wires connecting the backing to the board of the phone, so you don't have to worry about undoing any connectors.

6. The speaker for my earphone was not secured in my phone, so I was easily able to lift it up and clean some of the pocket lint from the inside of the grill and the front surface of the speaker (please be gentle with the speaker!). I recommend you only lift it out far enough to clean under and around it, then put it back.

7. When replacing the speaker, ensure you gently push it toward the electric ribbon connecting it to the board of you phone to ensure it fully seats in the recess designed for it.

8. At this point I cut a small piece of a rubber band, about 1 mm thick, and trimmed it to fit between the earpiece and the noise cancelation microphone, which on our t989 is integrated into the board and is not accessible. There is no way to remove and reseat it like the threads for the international version recommend. However, by placing the piece of rubber band between the earpiece and noise canceling mic they become more insulated from each other, which is a good thing. The rubber was not easy to place, so patience and a steady hand go a long way here. The rubber band is circled in red and labeled 1 in picture 3. The point where the backing snaps between the camera and flash is circled in green and labeled 2 in picture 3.

9. Next I cut a piece of a business card to fit the gap between the top of the phone and the trim surrounding the camera (see picture 4 for the trim). Make sure the card doesn't cover the gold contact as explained in one of the above threads. --- see how I'm kind of forcing you to read those? :) ---

--- The business card I used was 0.39mm thick. I didn't measure the other dimensions. If I ever go back in for any reason I will measure them and report here, but don't hold your breath. :) ---

10. I then cut a piece of business card for the lower microphone. This microphone is integrated into the board of the phone as well (see picture 6). It cannot be removed and reseated like the international version can. However, I decided to place a piece of business card as shown in picture 5 in hopes it would help dampen any vibrations in the vicinity that the microphone might have otherwise picked up.

--- I used the same business card at 0.39mm thick. I didn't measure this one either. ---

11. Place the card you cut for the lower mic so that it doesn't cover the solder points for the usb connector, nor sticks over the edge on the right in picture 5 (where the arrow points). The backing of the phone has a piece of hardware that resides in that recess, so don't overhang it.

12. Time to reassemble. Make sure the business cards (or whatever you decided to use) don't move from their positions when replacing the backing of your phone. Place the three tabs along the top edge of the backing into their recesses in the top of the main frame of your phone, then rotate the backing down onto the phones frame. Snap the edges into place, and be sure to press firmly on the camera cover between the camera and flash to seat the snap there. Don't force anything though. If your phone won't snap back together just remove the backing and make sure everything is situated properly (especially the earpiece speaker). When all the snaps are seated, reinstall your sim and battery and make a test call. If your phone cover makes echo worse, make sure it's on your phone. I even flashed an AOSP rom that notoriously gave me call echo.

13. If you're happy with the results, continue to step 14. If not, go back and rethink how you installed everything. Maybe you need to use a more sturdy business card, or cut it to different dimensions, or use a thicker or thinner card. Use your head and think it through. You can do it. Just be careful in there.

14. Congratulations! You did it! All you need to do is reinstall the six screws and you are done! I installed them in the exact opposite order as I removed them (starting with 6 and working to 1). Anyone who has installed a cylinder head on a car will know the reasoning behind this (to help prevent kinks or ripples in the material between the screws).

This all passed the wife test for me, and it helped muffle the sound of shifting my fingers on my phone. It no longer sounds like I'm beating it on a hard surface!

I really hope this works as well for you all as it has for me. Now, keep in mind that I have only made a few test calls since finishing this, but every person on the other end has said everything is crystal clear. I can now run AOSP ROM's on my Hercules!!! No tape over the noise cancellation microphone required!

Again, please go to the above threads and thank those guys. I take no credit for the fix. All I did was provide pictures specific to the Hercules (and maybe similar to Skyrocket????).

Enjoy!